Multispectral image capturing apparatus

ABSTRACT

A multispectral image capturing apparatus has different spectral sensitivity characteristics of at least four bands. Three primary bands of the at least four bands have spectral sensitivity characteristics of standard RGB. At least one auxiliary band of the rest of the at least four bands excluding the three primary bands has a spectral sensitivity characteristic of a narrower bandwidth than bandwidths of the RGB.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a Divisional Application of U.S. applicationSer. No. 11/416,939 filed May 3, 2006, now U.S. Pat. No. 7,612,822 whichis a Continuation Application of PCT Application No. PCT/JP2004/016678,filed Nov. 10, 2004, which was published under PCT Article 21(2) inJapanese, which is based upon and claims the benefit of priority fromprior Japanese Patent Application No. 2003-381419, filed Nov. 11, 2003,the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a multispectral image capturing apparatuscapable of capturing an image of at least four bands, each of which hasa different spectral characteristic.

2. Description of the Related Art

Conventionally, an RGB (three-band) camera has been widely used forcapturing images of a subject.

In recent years, to enable an image capturing apparatus to accuratelyreproduce colors of a subject, there has been proposed a method ofobtaining and recording more detailed spectral information of thesubject in the form of images by using a multispectral camera capable ofcapturing images of at least four bands. Methods relating to such animage capturing apparatus capable of capturing images of at least fourbands. Methods relating to such an image capturing apparatus capable ofcapturing images of at least four bands are disclosed in U.S. Pat. No.5,864,364; U.S. Pat. No. 6,466,334; Jpn. Pat. Appln. Publication No.2002-296114; Jpn. Pat. Appln. Publication No. 2003-023643; and Jpn. Pat.Appln. Publication No. 2003-087806, for example.

According to the methods disclosed in these documents, a filter having atransmittance characteristic of narrower bands than wavelengthbandwidths of RGB is placed in front of a CCD image capturing device toobtain the more detailed spectral information of the subject.

BRIEF SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provideda multispectral image capturing apparatus having different spectralsensitivity characteristics of at least four bands, comprising:

spectral sensitivity characteristics of three primary bands of the atleast four bands having spectral sensitivity characteristics of standardRGB; and

spectral sensitivity characteristic of at least one auxiliary band ofthe rest of the at least four bands excluding the three primary bandshaving a spectral sensitivity characteristic of a narrower bandwidththan bandwidths of the RGB.

According to a second aspect of the present invention, there is provideda multispectral image capturing apparatus, comprising:

a half mirror configured to divide light from an image capturing lensinto two light paths;

a band-pass filter configured to modulate a spectral characteristic ofone portion of the light divided by the half mirror;

an image capturing unit configured to receive the light modulated by theband-pass filter and capture an image of a subject; and

a color image capturing unit configured to receive the other portion ofthe light divided by the half mirror as three decomposed colors of red,blue, and green and capture a color image of the subject.

According to a third aspect of the present invention, there is provideda multispectral image capturing apparatus having different spectralsensitivity characteristics of at least four bands, comprising:

a dichroic mirror configured to have a comb-shaped spectraltransmittance characteristic and a comb-shaped spectral reflectancecharacteristic, transmit light of a plurality of wavelength bandwidths,and reflect light of bandwidths other than the plurality of wavelengthbandwidths;

a band-pass filter configured to have a transmissive wavelengthbandwidth substantially similar to a transmissive wavelength bandwidthof the dichroic mirror; and

a band-pass filter configured to have a spectral transmittancecharacteristic of transmitting light of a wavelength bandwidthsubstantially equal to a reflective wavelength bandwidth of the dichroicmirror.

According to a fourth aspect of the present invention, there is provideda multispectral image capturing apparatus, comprising:

half mirror means for dividing light from an image capturing lens meansinto two light paths;

band-pass filtering means for modulating a spectral characteristic ofone portion of the light divided by the half mirror means;

image capturing means for receiving the light modulated by the band-passfiltering means and capturing an image of a subject; and

color image capturing means for receiving the other portion of the lightdivided by the half mirror means as three decomposed colors of red,blue, and green and capturing a color image of the subject.

According to a fifth aspect of the present invention, there is provideda multispectral image capturing apparatus having different spectralsensitivity characteristics of at least four bands, comprising:

dichroic mirror means, having a comb-shaped spectral transmittancecharacteristic and a comb-shaped spectral reflectance characteristic,for transmitting light of a plurality of wavelength bandwidths andreflecting light of bandwidths other than the plurality of wavelengthbandwidths;

band-pass filtering means having a transmissive wavelength bandwidthsubstantially similar to a transmissive wavelength bandwidth of thedichroic mirror means; and

band-pass filtering means having a spectral transmittance characteristicof transmitting light of a wavelength bandwidth substantially equal to areflective wavelength bandwidth of the dichroic mirror means.

Advantages of the invention will be set forth in the description whichfollows, and in part will be obvious from the description, or may belearned by practice of the invention. Advantages of the invention may berealized and obtained by means of the instrumentalities and combinationsparticularly pointed out hereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate embodiments of the invention, andtogether with the general description given above and the detaileddescription of the embodiments given below, serve to explain theprinciples of the invention.

FIG. 1 is a diagram illustrating a configuration of a multispectralimage capturing apparatus according to a first embodiment of the presentinvention;

FIG. 2 is a graph representing a transmittance characteristic and areflectance characteristic of the half mirror used in the multispectralimage capturing apparatus according to the first embodiment;

FIG. 3 is a graph representing a spectral transmittance characteristicof the infrared light cut filter used in the multispectral imagecapturing apparatus according to the first embodiment, and spectralsensitivity characteristics of RGB three bands of the color CCD imagecapturing device used in the multispectral image capturing apparatus;

FIG. 4 is a diagram illustrating a configuration of pixels included inthe color CCD image capturing device used in the multispectral imagecapturing apparatus according to the first embodiment;

FIG. 5 is a graph representing a spectral transmittance characteristicof the band-pass filter used in the multispectral image capturingapparatus according to the first embodiment, and the spectralsensitivity characteristics of the RGB three bands of the color CCDimage capturing device used in the multispectral image capturingapparatus;

FIG. 6 is a graph representing a spectral transmittance characteristicof another band-pass filter that can be used in place of the band-passfilter of FIG. 5;

FIG. 7 is a graph representing spectral sensitivity characteristics ofsix bands obtained by the multispectral image capturing apparatusaccording to the first embodiment;

FIG. 8 is a diagram illustrating a configuration of a first modifiedexample of the multispectral image capturing apparatus according to thefirst embodiment;

FIG. 9 is a diagram illustrating a configuration of a second modifiedexample of the multispectral image capturing apparatus according to thefirst embodiment;

FIG. 10 is a diagram illustrating the four-component optical system ofFIG. 9 in which the half mirror is combined with the color separationprism;

FIG. 11 is a diagram illustrating a configuration of a multispectralimage capturing apparatus according to a second embodiment of thepresent invention;

FIG. 12 is a diagram illustrating a specific structure of a rotatingfilter shown in FIG. 11;

FIG. 13 is a diagram illustrating a corresponding relationship betweenthe state of the rotating filter and timing of image reading performedby the color CCD image capturing device;

FIG. 14 is a diagram illustrating a configuration of a multispectralimage capturing apparatus according to a third embodiment of the presentinvention;

FIG. 15 is a diagram illustrating an example of filter arrangement of (apart of) a mosaic filter installed in the color CCD image capturingdevice shown in FIG. 14;

FIG. 16 is a graph representing spectral transmittance characteristicsof the filters R, G, B, R′, G′, and B′ shown in FIG. 15;

FIG. 17 is a diagram illustrating another example of filter arrangementof (a part of) the mosaic filter installed in the color CCD imagecapturing device shown in FIG. 14;

FIG. 18 is a graph representing spectral transmittance characteristicsof the filters R′, G′, Cy, and B′ shown in FIG. 17;

FIG. 19 is a diagram illustrating a configuration of a multispectralimage capturing apparatus according to a fourth embodiment of thepresent invention;

FIG. 20 is a graph representing a spectral transmittance distributionand a spectral reflectance distribution of the dichroic mirror shown inFIG. 19, and spectral characteristics of RGB individually separated bythe color separation prism shown in FIG. 19;

FIG. 21 is a graph representing a spectral transmittance distribution ofthe band-pass filter 52 shown in FIG. 19, and the spectral transmittancedistribution of the dichroic mirror shown in FIG. 20;

FIG. 22 is a graph representing a spectral transmittance distribution ofthe band-pass filter 54 shown in FIG. 19, and the spectral reflectancedistribution of the dichroic mirror shown in FIG. 20;

FIG. 23 is a diagram illustrating a configuration of a multispectralimage capturing apparatus according to a fifth embodiment of the presentinvention;

FIG. 24 is a graph representing a spectral characteristic of a lightsource of the strobe illuminating light emitting unit used in themultispectral image capturing apparatus according to the fifthembodiment, and spectral sensitivity characteristics of RGB three bandsof the color CCD image capturing device used in the multispectral imagecapturing apparatus;

FIG. 25 is a diagram showing a timing chart illustrating a flow of animage capturing operation from shutter control to strobe light emissionand light receiving and image reading by the color CCD image capturingdevice; and

FIG. 26 is a diagram illustrating a configuration of a multispectralimage capturing apparatus according to a sixth embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION First Embodiment

Now, the present invention will be described below by referring to theaccompanying drawings that illustrate preferred embodiments of thepresent invention.

In a multispectral image capturing apparatus according to a firstembodiment of the present invention, as illustrated in FIG. 1, lightfrom a subject O is transmitted through a lens 10 and a half mirror (HM)12 and is formed into images on color CCD image capturing devices 14 and16. Transmittance and reflectance of the half mirror 12 used in thisexample are not equal in proportion. Their ratio is approximately threeto one. That is, as illustrated in FIG. 2, the half mirror 12 includes atransmittance 12A, which is constantly 75% irrespective of thewavelength, and a reflectance 12B, which is constantly 25% irrespectiveof the wavelength.

A larger (i.e., a transmission-side) portion of the light divided by thehalf mirror 12 is filtered through an infrared light cut filter (IR-CF)18 as shown in FIG. 3. Thereby, a near-infrared light is blocked, and aresultant light is formed into an image on the color CCD image capturingdevice 14. In FIG. 3, a solid line 18A indicates a spectraltransmittance of the IR-CF 18.

Meanwhile, a smaller (i.e., a reflection-side) portion of the lightdivided by the half mirror 12 is filtered through a band-pass filter 20,and a resultant light is formed into an image on the color CCD imagecapturing device 16.

As illustrated in FIG. 4, each of the color CCD image capturing devices14 and 16 is a single plate-type color CCD image capturing device inwhich color filters of R, G, B are arranged in the Bayer pattern forrespective pixels. The spectral transmittances of the respective filtersof R, G, B have spectral shapes as indicated by broken lines shown inFIG. 3.

Further, the band-pass filter 20 has a comb-shaped spectraltransmittance as indicated by a solid line 20A shown in FIG. 5.Therefore, the band-pass filter 20 transmits light of a bandapproximately half of each of the wavelength bandwidths of RGB indicatedby the broken lines in FIG. 5.

Alternatively, the band-pass filter 20 may have a comb-shaped spectraltransmittance as indicated by a solid line 20B shown in FIG. 6. Thespectral transmittance characteristic indicated by the solid line 20B isthe inversion of the spectral transmittance characteristic indicated bythe solid line 20A of FIG. 5. Therefore, with this band-pass filter 20having the spectral transmittance characteristic indicated by the solidline 20B, light of a near-infrared region can also be obtained.

With the configuration described above, an image of three bands havingspectral characteristics similar to spectral characteristics ofconventional RGB is obtained at the color CCD image capturing device 14,which is provided for the transmission-side portion of the light dividedby the above-described half mirror 12. Meanwhile, an image of threebands having spectral characteristics of narrower bandwidths than theconventional RGB is obtained at the color CCD image capturing device 16,which is provided for the reflection-side portion of the light.Therefore, the images of a total of six bands are obtained by the colorCCD image capturing devices 14 and 16. FIG. 7 illustrates spectralsensitivity characteristics of the total of six bands thus configured.As described above, the transmittance and the reflectance of the halfmirror 12 are unequal in proportion. Thus, in the spectral sensitivitiesof the six bands shown in FIG. 7, peak sensitivities of the latter threebands other than RGB are lower than peak sensitivities of the formerthree bands of RGB.

As described above, most of the incident light from the subject O isused to obtain the three bands of RGB, while the rest of the light of asmall amount is subjected to a bandwidth reduction by the band-passfilter 20 and then is allocated to the other three bands. By so doing,the loss of the light amount caused by the bandwidth reduction performedby the band-pass filter 20 is reduced as much as possible, anddeterioration of the sensitivity in multispectral image capturing isprevented. Accordingly, a spectral sensitivity characteristic with goodcolor reproducibility can be obtained.

Image signal obtained by the transmission-side color CCD image capturingdevice 14 is temporarily stored in a first image memory 22. Meanwhile,image signal obtained by the reflection-side color CCD image capturingdevice 16 is temporarily stored in a second image memory 24. Thethree-band images stored in the first image memory 22 and the secondimage memory 24 are synthesized into a six-band image at an imagesynthesizing unit 26 and stored in a storage medium and an external PC28. In this case, the image signals of the three bands of RGB can alsobe externally transmitted either directly from the transmission-sidecolor CCD image capturing device 14 or through the first image memory 22to be input in an RGB connecting device 30. Therefore, it is possible toconnect the multispectral image capturing apparatus to a commonly usedRGB monitor or the like and to use the RGB monitor as an image viewer.

The two color CCD image capturing devices 14 and 16 described in thepresent embodiment need not have the same resolution. For example, thereflection-side color CCD image capturing device 16 may be lower in theresolution than the transmission-side color CCD image capturing device14. Even though the image of the three auxiliary bands obtained by thereflection-side color CCD image capturing device 16 has a lowresolution, if high-frequency components in the image of the threeprimary bands obtained by the transmission-side color CCD imagecapturing device 14 are used, a multispectral image having a resolutionsubstantially equal to a resolution of the image of the three primarybands can be obtained. A low-resolution CCD image capturing device ishigher in sensitivity than a high-resolution CCD image capturing device.Thus, it is possible to decrease the reflectance of the half mirror 12and to increase the transmittance thereof. Accordingly, overallsensitivity can be further increased.

In the present embodiment, the bandwidth reduction is performed by theband-pass filter 20. However, the present invention is not limitedthereto. For example, the bandwidth reduction may be performed by otherways, such as performing the bandwidth reduction on transmissionspectral characteristics of the color filers included in the color CCDimage capturing device 16.

With reference to FIG. 8, a first modified example of the multispectralimage capturing apparatus according to the first embodiment will now bedescribed. In the present modified example, the above-described singleplate-type color CCD image capturing devices 14 and 16 shown in FIG. 1are replaced by three plate-type color image capturing units each ofwhich includes an RGB color separation prism and three CCD imagecapturing devices.

That is, one portion of the light divided by the half mirror 12 with anunequal intensity ratio is incident on a color image capturing unit 32through the IR-CF 18, and an image of the three bands of RGB is formedby the color image capturing unit 32. Meanwhile, the other portion ofthe light from the subject O is subjected to the bandwidth reductionperformed by the band-pass filter 20 which has the spectraltransmittance as shown in FIG. 5, and then is incident on a color imagecapturing unit 34. Then, an image of three bands is formed by the colorimage capturing unit 34. Thereby, image signals of a total of six bandsare obtained.

In this way, by using the three plate-type color image capturing units32 and 34, a multispectral image of a higher resolution and a highersensitivity can be obtained, compared with a case in which the singleplate-type color CCD image capturing devices 14 and 16 are used. Thethree plate-type color image capturing units 32 and 34 described aboveare widely used in image capturing apparatuses that capture movingimages, such as a conventional HDTV camera, and are effective incapturing multispectral images of moving pictures.

If an electronic shutter speed is individually determined for the CCDimage capturing device included in each of the color image capturingunits 32 and 34, images can be captured with optimal exposure.

FIG. 9 illustrates a second modified example of the multispectral imagecapturing apparatus according to the first embodiment. In the presentmodified example, the configuration of the first embodiment describedabove is combined with the configuration of the first modified example.

That is, out of one portion of the light divided by the half mirror 12with the unequal intensity ratio, light in the near-infrared region isblocked by the IR-CF 18. Thereafter, a resultant light is incident onthe color image capturing unit 32 in a similar manner as in the firstmodified example described above. Then, the light is separated into RGBlights by the color separation prism in the color image capturing unit32, and the RGB lights are formed into images on the respective CCDimage capturing devices. Thereby, a three-band image is captured.Meanwhile, the other portion of the light is subjected to the bandwidthreduction performed by the band-pass filter 20 and an optical low-passfilter (LPF) 36, and is formed into an image on the color CCD imagecapturing device 16 in a similar manner as in the first embodimentdescribed above. Thereby, a three-band image is captured. As the imagesare thus captured, image signals of the total of six bands are obtained.The optical LPF 36 used in this example is commonly used to decreasefalse colors and color moires. Therefore, although not illustrated inparticular in the drawings, the optical LPF 36 is similarly provided inthe first embodiment described above (and in the other embodiments laterdescribed).

As described above, in the second modified example, a multispectralimage of a high resolution and a high sensitivity can be obtained byusing the three plate-type color image capturing unit 32 for the threeprimary bands of RGB. Meanwhile, for the auxiliary three primary colors,an image is captured by the single plate-type color CCD image capturingdevice 16 at the cost of the resolution. As described above, however, amultispectral image of a resolution similar to the resolution obtainedby the conventional three-band HDTV camera can be obtained by using thehigh-frequency components in the image of the three primary bands.Accordingly, a small-size multi-band image capturing apparatus can beprovided.

In FIG. 9, the half mirror 12 and the color separation prism aredifferent elements, and the three bands at the transmission side and thethree bands at the reflection side are formed by separate units,respectively. However, the present invention is not limited thereto. Forexample, as illustrated in FIG. 10, it is possible to capture an imageof the six bands at one unit by using one device in which the halfmirror 12 is combined with the color separation prism 38, one sheet ofthe color CCD image capturing device 16, and three monochrome CCD imagecapturing devices 40R, 40G, and 40B. With this configuration, it ispossible to provide a six-band image capturing apparatus configured intoa further smaller size.

Second Embodiment

In a multispectral image capturing apparatus according to a secondembodiment of the present invention, as illustrated in FIG. 11, thelight from the subject C is transmitted through the lens 10 and arotating filter 42, and is formed into an image on one sheet of a colorCCD image capturing device 44. As illustrated in FIG. 12, the rotatingfilter 42 is formed by a transparent glass 42A having a transmittance ofapproximately 100%, and a band-pass filter 42B having the spectraltransmittance characteristic as shown in FIG. 5 described above. Therotating filter 42 is configured such that the area ratio between thetransparent glass 42A and the band-pass filter 42B is approximatelythree to one. As the light transmitted through the rotating filter 42 isfiltered through the IR-CF 18, a near-infrared light is blocked, and aresultant light is formed into an image on the color CCD image capturingdevice 44. Further, the rotating filter 42 is driven by a motor 46 torotate at a constant rotation speed, and the color CCD image capturingdevice 44 performs exposure twice during one rotation of the rotatingfilter 42.

FIG. 13 is a timing chart illustrating the state of the rotating filter42 and exposure timing of the color CCD image capturing device 44. Thefirst exposure is performed during a state in which the transparentglass 42A of the rotating filter 42 can be used, and a resultantthree-band image is stored in the first image memory 22. Further, thesecond exposure is performed during a state in which the band-passfilter 20 of the rotating filter 42 can be used, and a resultantthree-band image is stored in the second image memory 24. Then, therespective three-band images stored in the first image memory 22 and thesecond image memory 24 are synthesized at the image synthesizing unit26. Thereby, a six-band image is obtained.

With the above-described configuration, similar effects to the effectsof the first embodiment can be obtained by using a smaller number ofcomponents than the components of the first embodiment. Further, if thethree-band image stored in the first image memory 22 is input in the RGBconnecting device 30, a color image of the subject O can be easilychecked in a similar way as in the conventional RGB camera.

Third Embodiment

In a multispectral image capturing apparatus according to a thirdembodiment of the present invention, as illustrated in FIG. 14, thelight from the subject O is filtered through the IR-CF 18, so that thenear-infrared light is blocked. Then, an image is formed on one sheet ofthe color CCD image capturing device 44. In this example, as illustratedin FIG. 15, the color CCD image capturing device 44 is provided with amosaic filter 44A in which filters R, G, B, R′, G′, and B′ areperiodically arranged for the respective pixels. Spectral transmittancedistributions of the respective filters are shown in FIG. 16. That is,the filters R, G, and B have the same spectral bandwidths as theconventional three-band camera. Meanwhile, the filters R′, G′, and B′have narrower spectral bandwidths than the conventional RGB. In themosaic filter 44A, as illustrated in FIG. 15, the filters are arrangedat such a ratio that the filters R′, G′, and B′ are provided for onepixel while the filters R, G, and B are provided for three pixels, andthis set of arrangement is periodically repeated.

The light transmitted through the respective filters is received by thecolor CCD image capturing device 44 and sequentially input in an imageinterpolation processing unit 48 as image data. In the imageinterpolation processing unit 48, image data corresponding to a pixelposition lacking in each of the bands is obtained by interpolation onthe basis of nearby image data, and image data having the same pixelnumber is generated. Among the thus generated image data of therespective bands, an image of the three bands of RGB is sent to andstored in the first image memory 22, while an image of the three bandsof R′G′B′ is sent to and stored in the second image memory 24. If thethree-band image stored in the first image memory 22 is input in the RGBconnecting device 30, the color image of the subject O can be easilychecked in the similar way as in the conventional RGB camera. Further,it is possible to obtain a six-band image by synthesizing again therespective three-band images stored in the first image memory 22 and thesecond image memory 24 at the image synthesizing unit 26.

In this way, the narrow-band filters are supplementarily added to andmixed with the RGB filters having the same bandwidths as theconventional RGB, and a distribution ratio of pixels corresponding tothe RGB with respect to pixels corresponding to the auxiliary bands isincreased in acquisition of a multi-band point sequential image.Thereby, a multi-band image can be obtained with approximately equalresolution and sensitivity to the resolution and sensitivity of theconventional RGB camera. Accordingly, color reproducibility can beimproved.

In FIG. 15 described above, the pixels corresponding to the G′ filteramong the supplementarily added R′, G′, and B′ filters are providedtwice as many as the pixels corresponding to the other filters of R′ andB′. However, the present invention is not limited thereto. For example,as illustrated in FIG. 17, one of the two pixels corresponding to the G′filter may be allocated to a Cy filter so that a multispectral imagehaving a total of seven bands is captured. In this case, the Cy filterhas a different spectral transmittance characteristic from the spectraltransmittance characteristic of the above-described G′ filter.

In the first and second embodiments described above, the three auxiliarybands other than the conventional RGB three bands have bandwidths ofpredetermined ranges extracted from the bandwidths of the original RGB.However, the present embodiment is not limited thereto. For example, theauxiliary bands can include a bandwidth beyond the wavelength bandwidthsof the original RGB, as in the case of Cy shown in FIG. 18.

Fourth Embodiment

In a multispectral image capturing apparatus according to a fourthembodiment of the present invention, as illustrated in FIG. 19, adichroic mirror (DM) 50 that transmits and reflects light with aselected wavelength is used in place of the half mirror 12 used in thepreceding embodiments, which divides the light from the subject O. InFIG. 20, a spectral transmittance distribution of the DM 50 is indicatedby a black fine line 50A, while a spectral reflectance distribution ofthe DM 50 is indicated by a black dotted line 50B. Further, in FIG. 20,black bold lines indicate spectral characteristics of RGB individuallyseparated by the color separation prism shown in FIG. 19. As illustratedin FIG. 20, the DM 50 has the transmittance distribution and thereflectance distribution each of which has a comb-like spectral shape.

Then, the light transmitted and reflected by the DM 50 is decomposed bythe color separation prisms of the color image capturing units 32 and 34into the RGB three bands of different narrow bandwidths, and are formedinto images. Thereby, an image having six bands of the narrow bandwidthsis obtained.

Further, band-pass filters 52 and 54 are inserted at positionssubsequent to the DM 50. The band-pass filters 52 and 54 have a spectraltransmittance 52A and a spectral reflectance 54A as shown in FIGS. 21and 22, respectively, which have comb-like spectral shapes similar tothe spectral transmittance 50A and the spectral reflectance 50B of theDM 50. Thus, wavelength selectivity is intensified, and therefore, thesix-band image with good wavelength separation can be obtained. Ingeneral, if an attempt is made to obtain transmittance and reflectancedistributions having comb-like spectral shapes by solely using thedichroic mirror 50, wavelength selectivity cannot be much expected.Thus, as illustrated in FIG. 20, light leakage of a certain amount iscaused in a non-transmission band and a non-reflection band in manycases. Therefore, improving the wavelength separation of the respectivebands by placing the band-pass filters 52 and 54 of good wavelengthselectivity (e.g., multilayer-film interference filters) at thepositions subsequent to the DM 50, as described above, is a veryeffective technique for providing a multispectral camera having goodcolor reproducibility. Further, the light loss caused by the band-passfilter 20 can be minimized by separating the wavelengths of the lightfrom the subject O to some extent by the DM 50. Accordingly, amulti-band camera with high light efficiency can be provided.

With the above-described configuration, according to the presentembodiment, the image having the six bands of the narrow bandwidths iscaptured, and therefore, a multispectral image with good wavelengthseparation can be captured. Further, a multi-band camera of a highsignal-to-noise ratio can be provided. However, unlike the precedingembodiments, the multispectral image capturing apparatus according tothe present embodiment cannot be directly connected to the conventionalRGB connecting device 30 to check the color image equal to the ordinaryRGB image. Therefore, to check the obtained image as a color image, itis necessary to perform a color conversion on the obtained image byusing a multi-band image processing device 56 as shown in FIG. 19.

As described above on the basis of the first to fourth embodiments,among the bands used for capturing a multispectral image, at least threebands have the spectral sensitivity characteristics of wide bandwidthssimilar to the conventional RGB. Further, a smaller portion of the lightfrom the subject than the portion of the light allocated to the RGB isallocated to the other auxiliary bands of the narrow bandwidths.Thereby, the light loss is reduced, and thus it is possible to provide amultispectral image capturing apparatus capable of capturing images ofthe subject O with good color reproducibility, even if the subject O ismoving. Furthermore, it is possible to provide a multispectral imagecapturing apparatus capable of directly checking an obtained image byusing an ordinary RGB monitor with no need for a special converter.Moreover, if the dichroic mirror and the band-pass filter are usedtogether, the six bands of narrow bandwidths with good wavelengthseparation can be obtained, and a multispectral image capturingapparatus that reduces the light loss can be provided, similarly to theabove.

Fifth Embodiment

As illustrated in FIG. 23, a multispectral image capturing apparatusaccording to a fifth embodiment of the present invention includes astrobe illuminating light emitting unit 58 which includes a light sourcehaving the wavelength characteristics of three band of narrowbandwidths. Therefore, two types of images (each having three bands),i.e., an image captured with no strobe light emission (i.e., withambient lighting) and an image captured with the strobe light emission(i.e., with ambient lighting and strobe lighting) are captured, and thena multispectral image of six bands is obtained. In this case, the lightsource of the strobe illuminating light emitting unit 58 is a lightsource, such as a white LED and fluorescent light, which has awavelength characteristic of a comb-shaped spectral distributionincluding sharp peaks in the wavelength regions of the sensitivities ofRGB included in the color CCD image capturing device 44. Alternatively,three types of LEDs having the wavelength characteristics of narrowbandwidths may be used. An example of an emission spectrum of the lightsource of the strobe illuminating light emitting unit 58 is indicated bya solid line in FIG. 24. In the figure, broken lines indicatetransmittance characteristics of RGB.

The multispectral image capturing apparatus according to the presentembodiment further includes an image subtracting unit 60 which subtractsthe image stored in the first image memory 22 from the image stored inthe second image memory 24.

FIG. 25 is a timing chart illustrating the flow of an image capturingoperation from shutter control to strobe light emission and lightreceiving and image reading by the color CCD image capturing device 44.That is, in the multispectral image capturing apparatus according to thepresent embodiment, when a shutter 62 is first pressed, the presentmultispectral image capturing apparatus captures an image of the subjectO with no strobe light emission, i.e., only with ambient lighting, as anordinary color image having the three bands of RGB. Then, the obtainedimage is stored in the first image memory 22. The image is then outputto the RGB connecting device 30 as the conventional RGB image and isoutput to the storage medium and the external PC 28 as an image of threeprimary bands among the six bands. Thereafter, the subject O isilluminated in the strobe light emission state, i.e., with the ambientlighting and the strobe lighting, and the image of the subject O iscaptured in a similar manner as the three-band color image, and theobtained image is stored in the second image memory 24. After the imagecapturing operation is completed, at the image subtracting unit 60, theimage previously captured with no strobe light emission (i.e., only withthe ambient lighting) and stored in the first image memory 22 issubtracted from the image stored in the second image memory 24. Thereby,a three-band image of the narrow bandwidths containing only strobecomponents is generated. The thus generated image is output to thestorage medium and the external PC 28 as an image of the three auxiliarybands. As a result, an image of a total of six bands including thethree-band image containing ambient components and the three-band imagecontaining the strobe components is obtained.

With the configuration as described above, similar effects to theeffects of the preceding embodiments can be obtained by using anordinary optical system, without using the special optical system or thefilter structure used in the preceding embodiments. Further, if thethree-band image stored in the first image memory 22 is input in the RGBconnecting device 30, the color image of the subject O can be easilychecked in a similar way as in the conventional RGB camera.

In the present embodiment, after the shutter operation is performed, thecolor image of the subject O is first captured with the light source ofonly the ambient light. Alternatively, the strobe light emission may befirst performed to capture the color image of the subject O illuminatedby the ambient light and the strobe light and then to capture the colorimage of the subject O with the light source of only the ambient light.

Sixth Embodiment

A multispectral image capturing apparatus according to a sixthembodiment of the present invention is a dual-lens multispectral imagecapturing apparatus. The multispectral image capturing apparatusincludes a color image capturing unit that captures an image of theconventional three bands of RGB, and a color image capturing unit thatcaptures an image of three bands of narrow bandwidths and includes alens provided with a band-pass filter having a comb-shaped spectraltransmittance characteristic of the three bands of the narrowbandwidths. Thus, the multispectral image capturing apparatus obtains asix-band multispectral image from the image of the three primary bandscaptured by the color image capturing unit that captures the image ofthe conventional three bands of RGB, and the image of the threeauxiliary bands captured by the color image capturing unit that capturesthe image of the three bands of the narrow bandwidths.

An image capturing unit at an upper position in FIG. 26 is the colorimage capturing unit that captures the image of the conventional threebands of RGB, and the color image capturing unit includes the lens 10,the IR-CF 18, and the color CCD image capturing device 14. Meanwhile, animage capturing unit at a lower position in FIG. 26 is the color imagecapturing unit that captures the image of the three bands of the narrowbandwidths, and the color image capturing unit includes a lens 64, theband-pass filter 20 placed in front of the lens 64 and having thecomb-shaped spectral transmittance characteristic of the three bands ofthe narrow bandwidths, and the color CCD image capturing device 16. Theposition of the image of the three auxiliary bands of the narrowbandwidths captured by the lower image capturing unit is misaligned fromthe position of the conventional three-band image (captured by the upperimage capturing unit) due to a parallax error. This misalignment iscorrected by a geometry correcting unit 66 which is provided at aposition subsequent to the second image memory 24, and a resultant imageis output after the positions of the two three-band images have beenaligned to each other.

With the configuration described above, similar effects to the effectsof the preceding first to fourth embodiments can be obtained by using anordinary optical system, without using the special optical system or thefilter structure used in the first to fourth embodiments. Further, ifthe three-band image stored in the first image memory 22 is input in theRGB connecting device 30, the color image of the subject O can be easilychecked in a similar way as in the conventional RGB camera.

Needless to say, the present invention is not limited to theabove-described embodiments, but can be modified or applied in variousways within a scope of the gist of the present invention.

For example, the CCD image capturing device used in the above-describedembodiments as an example of the image capturing device can be replacedby an X-Y address type image capturing device, such as a CMOS imagecapturing device. If such an image capturing device is used, high-speedreading and skip reading can be performed. Further, if a single-platetype color image capturing device is used, pixels corresponding to thesame color filter can be selectively read. Therefore, high-speedprocessing and an inexpensive structure of the multispectral imagecapturing apparatus are enabled.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details, and representative devices shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1. A multispectral image capturing apparatus, comprising: a color imagecapturing unit configured to capture a color image of a subject; afilter arranged opposite to the color image capturing unit, configuredto make light of standard RGB from the subject incident on the colorimage capturing unit at a predetermined timing, so that the color imagecapturing unit captures an image of main three bands having spectralsensitivity characteristics of the standard RGB, of spectral sensitivitycharacteristics of six bands, and configured to make light of auxiliarythree bands incident on the color image capturing unit at a timingdifferent from the predetermined timing, the auxiliary three bandscorresponding to remaining bands other than the main three bands of thespectral sensitivity characteristics of the six bands, and each of theauxiliary three bands having spectral sensitivity characteristics in abandwidth narrower than a bandwidth for the RGB, so that the color imagecapturing unit captures an image of the remaining bands other than themain three bands of the spectral sensitivity characteristics of the sixbands; a synthesizing unit configured to synthesize the image of themain three bands and the image of the auxiliary three bands as capturedat the different timings by the color image capturing unit, to generatean image of the six bands; and an output unit configured to output theimage of the main three bands captured by the color image capturingunit.
 2. The apparatus according to claim 1, wherein the filtercomprises a band-pass filter configured to modulate a spectralsensitivity characteristic of light from the subject to make the lightof the auxiliary three bands incident on the color image capturing unit.3. The apparatus according to claim 2, wherein the filter comprises arotary filter provided with a transparent glass for making the light ofthe standard RGB incident on the color image capturing unit and theband-pass filter.